The inventive apparatus is utilized, for example, in Raman spectroscopy. The light that is to be analyzed is introduced through an entry slit positioned at an off-axis focal point of a collimator that consists of a spherical mirror. The mirror reflects a parallel pencil of light to a plane diffraction grating. The grating diffracts the light and reflects it in several parallel pencils to another spherical concave mirror, which acts as a focusing mirror. The result is separate images of the entry slit for each wavelength of incoming light. It is then possible, when the apparatus is employed as a spectrograph, to record or observe the whole resulting spectrum or, when the apparatus is employed as a monochromator, to isolate a single wavelength at one exit slit.
The collimating and focusing mirrors in an apparatus of this type are employed off axis and produce aberrations, especially spherically aberrations, which may also be accompanied by coma and astigmatism.
The object of the present invention is to reduce these aberrations as much as possible by correcting them by means of a particular arrangement of the lines on a corrected plane grating in such away that the lines will exhibit very slight deviations in parallelism and pitch. The corrected grating is a holographic grating, and another object of the invention is accordingly the method employed to holographically record it.